Perceptual Processes

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How to get beyond intuition?

Step 1: Distinguish the effect from its intepretation.

Michotte: the experience of launching depends on interactions among various factors including

the relative speeds of the two objects
the delay between the first and second objects’ movements
the spatial gap between the two objects
the trajectories of the two objects.

But how does this help us? Importantly, tiny variations in the parameters will make big differences in the experiences reported. Let me illustrate this for the delay between the objects' movements.

adults: (b) they can discriminate between short gaps and long gaps.

That is, the can discriminate gaps of around 50ms.

Michotte 1946 [1963], p. 115 table IX (part)

Maybe this is clearer as a figure.

drawn from Michotte 1946 [1963], p. 115 table IX (part)

People can distinguish between stimuli that differ only in that the gap between two movements is approximately 50ms longer in one than the other. A 50ms difference makes the difference between reporting launching and reporting two movements.

We need to do more to understand the effect, ...

The question was how we can get beyond intuition in understanding the verbal reports.

Part of the answer is this. We don't worry about the content of the verbal reports. We just focus on the fact that their content changes depending on a tiny, 50 millisecond difference in the delay between two movements. Call this \emph{launching effect}.

- The launching effect: a 50ms difference in the delay between two movements changes subjects’ judgements, ...

This doesn't tell us \emph{what} people are detecting. But it does tell us that the effect is not merely confabulation or making it up. So we have taken a tiny step beyond intuition. But we also have to answer two questions.

How to get beyond intuition?

Step 1: Distinguish the effect from its intepretation.

Step 2: Which processes underpin the effect?

So we have the launching effect: adults and probably infants too exhibit perceptual sensitivity to differences in timing of around 50 milliseconds, but only when such delays make the difference between a causal interaction or non-causal interaction.

We are still trying to understand the nature of the launching effect. To make progress we need to think about how it arises.

Guess how the launching effect works! A natural thought is this: first you perceive objects, then you identify causal interactions based on contiguity etc. This turns out to be completely wrong.

Guess how the launching effect works!

The impression of launching is judgement-independent. So it can't be a consequence of thinking about the interaction. Still, it might be a consequence of perceiving objects in certain relations to each other. However a key finding shows that this is wrong. Surprisingly, we don't first perceive objects and then get the launching effect; rather, the launching effect is tied up with perceptual process of identifying objects' surfaces.

judgement-independent

Thines et al (1991)

[I'm about to talk about illusory causal crescents. I first show them two videos. This is a full overlap video. You can drag the slider to show them that it's full overlap, but first ask them what they see.] [Static images follow in case video doesn't work.]

what did you see?

Normally, if the two balls overlap completely, subjects report seeing a single object changing colour.

[This is a causal capture video with full overlap. Focus on the top sequence. Tell me what you saw! You can drag the slider to show them that it's full overlap, but first ask them what they see.] [Static images follow in case video doesn't work.]

what did you see?

Normally in this case people report the impression that the top sequence collided. That is, they didn't pass, they collided.

causal capture

Causal capture is described by \citep{Scholl:2002eb}. As I said, normally, if the two balls overlap completely, subjects report seeing a single object changing colour. But if we show subjects a sequence like the launching effect but where the first square overlaps the second's position before it moves. When this event is shown is isolation almost all subjects see it as a single object changing colour. But when the event is shown with an unambiguous launching effect nearby, almost all subjects now see the 'overlap' event as a launching. Causal capture means that we can show subjects a sequence with complete overlap and still have the report a causal effect.

Why do we care about causal capture? Because it gives rise to illusory causal crescents ...

[Now I'll explain illusory causal cresecents.]

Scholl and Nakayama 2004, figure 2 (part)

Here's a static image representing the sequence you saw first, when there was full overlap.

‘when there is a launching event beneath the overlap (or underlap event) timed such that the launch occurs at the point of maximum overlap, observers inaccurately report that the overlap is incomplete, suggesting that they see an illusory crescent.’ \citep[p.\ 461]{Scholl:2004dx}

Why does the illusory causal crescent appear? Scholl and Nakayama suggest a ‘a simple categorical explanation for the Causal Crescents illusion: the visual system, when led by other means to perceive an event as a causal collision, effectively ‘refuses’ to see the two objects as fully overlapped, because of an internalized constraint to the effect that such a spatial arrangement is not physically possible. As a result, a thin crescent of one object remains uncovered by the other one-as would in fact be the case in a straight-on billiard-ball collision where the motion occurs at an angle close to the line of sight.’ \citep[p.\ 466]{Scholl:2004dx}

*here or later? Contrast Spelke’s view. ‘objects are conceived: Humans come to know about an object’s unity, boundaries, and persistence in ways like those by which we come to know about its material composition or its market value.’ \citep[p.\ 198]{Spelke:1988xc}.

Scholl and Nakayama 2004, figure 5

(*This figure just shows when the overlap event was perceived as causal.)

Scholl and Nakayama 2004, figure 4

(*This figure shows the illusory causal crescents -- the underestimation of overlap.)

How to get beyond intuition?

Step 1: Distinguish the effect from its intepretation.

Step 2: Which processes underpin the effect? Perceptual processes!

What does this tell us about the emergence of knowledge of causal interactions in development?

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Perceptual Processes

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